In electric power plants operating with fossil fuels, the flue gases contain fly ash and other solids as well as acidic gaseous components such as sulfur oxides which must be scrubbed or otherwise removed from the gases before they are released into the atmosphere to prevent environmental pollution.
It is a common practice to provide downstream of an electric power plant boiler apparatus for the wet removal of dust and for the scrubbing of sulfur oxide from the waste or flue gas. The wet washing and scrubbing or desulfurization process produces a sludge of characteristic composition associated with the use of fossil fuels; this sludge presents a problem of disposal.
In this connection, it is known to pass the exhaust gases into a tower with or without annular-gap washers for the simultaneous dust removal and desulfurization of the gas using a scrubbing liquid, generally water. Water-soluble components of the gas stream saturate the scrubbing water and particulates are entrained from the gas stream in water droplets which are collected in a water-gas separator or the like. The process can include the condensation of water as part of the separation operation and the collected liquid with the entrained particulates can be conducted to a sedimentation tank in which a sludge settles from the excess liquid, which, in turn, may be recirculated to a scrubber.
While a certain proportion of the sulfur oxides are removed by solubilization in water, it is a conventional process to include the removal of the sulfur oxides by introducing into the scrubbing finely divided alkaline additives, such as particles of lime (calcium oxide) which form condensation nuclei about which condensates form, the alkaline additives reacting with the sulfur oxide to produce, for example, the corresponding calcium salts. The latter are present in the sludge.
Power plant boilers also produce so-called "melting-chamber slag" or scale or ash formed by fusion in the combustion chamber which must periodically be removed. Elimination, on the one hand of the aforementioned sludge and, on the other hand, the slag is a problem if environmental pollution is to be avoided.
It has been proposed heretofore in, for example, German patent DT-PS 1,817,001 to recover the sludge from the dust scrubbing and desulfurization devices of a power plant and to mix the sludge with molten melting-chamber slag to granulate the latter and simultaneously dewater the sludge with the sensible heat of the slag. The resulting solid mixture is used as an aggregate for concrete after being milled. Experiments have shown that such an aggregate does not produce a concrete of high-compressive strength, or for that matter, a concrete which for a given compressive strength, permits a reduction in the amount of the hydraulic binder which is employed.
It has also been proposed to use broken melting-chamber slag as an additive or aggregate for concrete (see German patent DT-PS 1,176,547 and German patent application DT-AS 1,228,984). Even with these systems, however, for a given proportion of hydraulic binder, e.g. portland cement, the concrete has only its customary compressive strength. The utility of the aggregate is thus limited even for those functions for which it has been found to be advantageous, namely, structures in which thermal conductivity should be limited.
Finally, it has been proposed to react a sludge obtained by the water-scrubbing desulfurization of flue gases with the dust obtained by a dry dust removal operation to produce a stone-like aggregate (open German application DT-OS 24 00 350). While these granulates can be used as an aggregate for concrete, this aggregate does not improve the compressive strength thereof.